1. Field of the Invention
The present invention relates to valve assemblies which control the flow and pressure of fluid to hydraulically power a machine; and more particularly to pressure compensated valves wherein a fixed differential pressure is to be maintained to achieve a uniform flow rate.
2. Description of the Related Art
Agricultural, construction and industrial machinery have components that are moved by hydraulic actuators, such as cylinder and piston arrangements. Application of hydraulic fluid to the hydraulic actuator is often controlled by a valve with spool that is moved by a manually operated lever or an electric solenoid. Movement of the spool into various positions within a valve body proportionally varies the flow of pressurized fluid from a pump to one chamber of the cylinder and controls fluid draining from another cylinder chamber. Typically a plurality of valves for operating different hydraulic actuators were combined side by side in sections of a larger valve assembly.
The speed of a hydraulically driven component on the machine depends upon the cross-sectional areas of control orifices in the spool valve and the pressure drop across those orifices. To facilitate control, pressure compensating hydraulic control systems have been designed to set and maintain the pressure drop. These previous control systems include load sense lines which transmit the pressure at the valve workports to the input of a variable displacement hydraulic pump which supplies pressurized hydraulic fluid in the system. The resulting self-adjustment of the pump output provides an approximately constant pressure drop across a control orifice, the cross-sectional area of which is varied by the machine operator. This facilitates control because, with the pressure drop held constant, the speed of the machine component is determined only by the cross-sectional area of an operator variable metering orifice.
One such prior system is disclosed in U.S. Pat. No. 5,579,642 entitled “Pressure Compensating Hydraulic Control System”. That system utilized a chain of shuttle valves to sense the pressure at every powered workport of each valve section and to choose the highest of those workport pressures, as a “load sense pressure”. The resultant load sense pressure was applied to an isolator valve which connected the control input of the pump to either the pump output or to the system tank depending upon that workport pressure. The isolator valve was contained in a separate, special end section of the valve assembly.
The control pressure applied to the pump's control input also was applied to a separate pressure compensating valve located in each valve section between the metering orifice of the control valve and the load being driven. This arrangement was referred to a “post-pressure compensated hydraulic control valve” because the compensation was located after, or downstream of, the metering orifice. The pressure compensating valve responded to the control pressure by creating a substantially fixed differential pressure across the spool. When the flow demand for a valve section exceeded the available flow supply, the pressure compensating valve in the valve sections split the available flow among the valve sections in proportion to the metering orifices in the respective spools.
In the prior post-pressure compensation technique, the pressure compensating valve in every valve section received the same control signal that was derived from the load sense signal. For certain machines, however, it is desirable to limit individually the load sense pressure controlling the pressure compensating valve in selected valve sections. Heretofore individual limiting was difficult to accomplish in a post-pressure compensation system because limiting the load sense pressure signal in one valve section often affected all the valve sections.